Containers which were conventional in the past, of white or colored sheet metal, of glass or also ceramic are being increasingly replaced by plastic containers. Mainly plastic containers are used for packaging of fluid substances, for example beverages, oil, cleaning products, cosmetics, etc. The low weight and lower costs certainly play a not inconsiderable part in this substitution. The use of recyclable plastic materials and the altogether more favorable overall energy balance in their production also contribute to promoting the acceptance of plastic containers, especially plastic bottles, among consumers.
The production of plastic containers, especially plastic bottles, for example from polyethylene or polypropylene, takes place in an extrusion blow molding process, especially in an extruded tube blowing process. In this connection a plastic tube is extruded from an extruder head, placed in the blow molding tool arrangement, inflated by overpressure via a blow mandrel, and cured. The extrusion blow molding machines used for this purpose generally have at least one extruder for supplying the plastic material. The output of the extruder is connected to the extruder head, and a tube extruded in one or more layers emerges on its exit nozzle which can be adjusted preferably in opening width. The extruded plastic tubing can have one or more layers, and can be extruded as tubing with visible strips, decorative strips or relative to the periphery with several, preferably variously colored segments. The extruded tubing is transferred to the blow molding tool arrangement and inflated by overpressure using a blow mandrel which is inserted into the mold cavity. Afterwards the inflated plastic container is removed from the mold cavity.
The number of plastic containers which can be produced per unit of time and per cycle directly influences the economic efficiency of the production process, the manufacturing costs of the individual containers and thus the amortization of the blow molding tool and the production facility. The effect of the quantitative factor is greater, the smaller the plastic container to be produced. For this reason extrusion blow molding systems are known in which the blow molding tool arrangement has one double cavity per extrusion nozzle. The double cavity consists of two mold cavities which are arranged axially under one another and which are connected to one another at the mouths of the containers to be inflated. The blow mandrel which is guided in a hole of the blow molding tool can be laterally fed to the region of the mutually adjoining mouths of the blow mold cavities. The blow mandrel penetrates the wall of the plastic tubing in a middle region which connects the two mouths and inflates two containers in one cycle from the plastic tubing which has been inserted into the double cavity. The finished-inflated containers are then removed from the blow mold cavities.
To remove the finish-inflated plastic containers, especially plastic bottles, from the mold, there are ejectors in the blow molding tool arrangement. The ejectors are located in the peripheral regions of the blow molding tool arrangement and generally act in one section of the plastic container which borders its bottom region. In this connection it can happen that the ejectors leave impressions in the wall of the not yet completely cured plastic containers. These impressions are however undesirable on plastic containers. On the one hand they adversely influence the appearance of the plastic containers. On the other hand the impressions, depending on their depth and the wall thickness of the plastic containers, can constitute unwanted weakening regions.